Glossary of terms

Alternate host: second host species required by some rusts and other organisms to complete their life cycle.

Auricle: pale-coloredpapery extension of a leaf blade at its junction with its leaf sheath. Also called a collar.

Auxin: compound regulating the growth of plants.

Chlorophyll: green pigment in plants required for photosynthesis.

Chloroplast: plant structure where photosynthesis occurs.

Chlorosis: abnormal yellowing (or whitening) of normally green leaves.

Collar: another word for the ligule and auricle grouped together.

Conidiophore: threadlike stalk upon which conidia (spores) are produced.

Conidium (pl. conidia): any asexual spore formed on a condiophore.

Conventional tillage: inverting the soil surface layer, incorporating crop residues and vegetation, and breaking up the suface to a fine tilth.

Crusting: sealing of the soil surface by fine soil particles. These block the larger pores, reducing the movement of gases into and from the soil and reducing the penetration of water.

Diapause: a period of dormancy.

Double ridges: a developmental stage of the growing apex (growing point) when it is approximately 0.5 mm long; the new leaf primordia are rounded ridges and associated with each one is a bud in its axil.

Enzyme: an organic compound catalyzing a specific reaction in the cell.

Exudate: gel-like accumulation of spores or bacterial ooze.

Foot rot: disease symptoms, such as discoloration, necrosis and decay, affecting the roots and basal portions of the plant or culm.

Gall: a localized proliferation of plant or parasite tissue that produces an abnormal growth or swelling, usually caused by pathogenic organisms, nematodes, or insects.

Green leaf area index (GLAI or LAI): the area of all green leaves on a square meter of ground; when each 1 square meter of ground has more than 4 square meters of leaf growing on it, the crop is intercepting about 90% of the solar radiation falling on it. GLAI is well linked to yield.

Heading: developmental stage when the head, spike or ear emerges from its enclosing sheath.

Honeydew: sticky exudate (containing conidia) produced during one stage of the life cycle of Claviceps purpurea.

Hypha (pl. hyphae): a tubular, threadlike filament of fungal mycelium.

Immune: not affected by pathogens.

Inoculum: spores or other diseased material that may cause infection.

Lesion: a visible area of diseased tissue on an infected plant.

Ligule: papery structure 1 mm or so which marks the junction between a leaf sheath and its blade and is in the same plane as the sheath; together with the auricle is it called the leaf collar.

Meristem: tissue of rapidly dividing cells, generally at the apex of shoot and root.

Metabolite: compound undergoing chemical transformations within the plant.

Micronutrient: nutrient required in trace amounts.

Minimum tillage: with a limited number of passes of machinery, it aims to achieve some soil disturbance and physical weed control but to leave much of the crop residues on the surface of the soil or in the surface layers.

Mosaic: a pattern of disease symptoms displaying mixed green and lighter colored patches.

Mycelium (pl. mycelia): a mass of hyphae that form the body of a fungus.

Necrosis: abnormal death of leaves, usually accompanied by discoloration.

Pan evaporimeter: an aboveground circular metal trough about 1.5 m diameter and 30 cm deep and filled with water. The number of mm of water evaporated from the pan indicates approximately how much water the crop is losing, provided there is no water stress. It can be a guide to how much irrigation water needs to be applied to replace transpiration losses.

Pathogen: a microorganism that causes disease.

Perennial weeds: plants that live for several years; the most difficult perennial weeds to control are those that propagate both by seeds and by vegetative means.

Phloem: specialized plant tissue mainly for conducting organic substances.

Photosynthesis: process used by plants to make sugars (carbohydrates) for growth. Essential ingredients in the process are solar radiation to provide energy, the green chlorophyll of the leaves and shoots to trap and convert that energy, carbon dioxide from the air to provide the carbon in the carbohydrate, and water.

Primary inoculum: spores or fragments of a mycelium capable of initiating a disease.

Pustule: a spore mass developing below the epidermis, usually breaking through at maturity.

Reductase: an enzyme that catalyzes reduction.

Resistance: inherent capacity of a host plant to prevent or retard the development of an infectious disease.

Sclerotium (pl. sclerotia): a dense, compact mycelial mass capable of remaining dormant for extended periods.

Senescence: phase of plant growth that extends from full maturity to death; the normal death of leaves.

Solar radiation: the energy from the sun that reaches the ground, remaining after passage through the atmosphere and clouds. After half that energy is visible light. Plants can use about half for photosynthesis.

Spore: a minute reproductive unit in fungi and lower plant forms.

Sporulation: period of active spore production.

Stem node: where a leaf arises on the stem; can be felt as bumps or joints when the finger and thumb are run up the stem. Internodes are the regions between the bumps.

Stephenson Screen: a white wooden box with angled slatted walls allowing through ventilation but no direct light, mounted about 1.5 m above the ground in which meteorological instruments are kept. Instruments are usually for measuring humidity and maximum and minimum temperatures.

Stomata: the active pores on both surfaces of leaves in wheat, that, by changing their apertures, control the rate at which carbon dioxide enters and water vapor exits the plant.

Striate: displaying narrow parallel streaks or bands.

Susceptible: being subject to infection or injury by a pathogen; non-immune.

Symptom: a visible response of a host plant to a pathogenic organism.

Telium (pl. telia): pustule containing teliospores.

Teliospore: thick-walled resting spore produced by rust and smut fungi.

Tolerant: the ability of a host plant to develop and reproduce fairly efficiently while sustaining disease.

Transmission: the spread of a disease agent among individual hosts.

Transpiration: leaves are covered in stomatal pores; when these pores are open, water vapor leaks through them from the inside of the leaf to the air, a process called transpiration.

Urediospore: an asexual spore of the rust fungi.

Vector: organism capable of transmitting inoculum.

Virulence: the relative ability of a microorganism to overcome the resistance of a host.

Water soaked: appear wet, darkened, and partially transparent.

Water use efficiency (WUE): the amount of material produced by the crop for each unit of water used. It may also be expressed as the amount of water required to produce biomass or grain.

Withertip: death of the leaf beginning at the tip, usually in young leaves.

Xylem: specialized plant tissue for conducting water and inorganic salts from roots to leaves.

Zero tillage: also called direct drill. In one pass the machinery cultivates a row slot, drills the seed, and places the fertilizer in the slot and covers the seed. Relatively little of the previous crop residues are disturbed or incorporated.